Cables known as tip cables are used between a central office cable entrance facility and the wire terminating frames in a central office. Tip cables are designed to be fire-resistant and are typically jacketed with fire-resistant and low-smoke producing sheaths. A commonly used tip cable, which comprises a cable core with a plurality of insulated conductors surrounded by a plastic core wrap, has a protective sheath known as the ALVYN sheath. The ALVYN sheath comprises an overlapped, corrugated aluminum shield surrounding the cable core and a relatively thin extruded polyvinyl chloride (PVC) outer jacket.
The tip cables are spliced in vault closures in the cable entrance facility to feeder cables entering the central office from the outside plant. The vault closures must be waterproof and gas tight to prevent entrance of water.
While the ALVYN sheaths of the tip cables are desirable for their fire-resisting properties, they are relatively soft and have less desirable mechanical properties, making the tip cables difficult to seal at their entry into a vault closure. One problem is that if much pressure is applied by a sealing member onto the tip cable, the shield will circumferentially slip over itself, causing the PVC jacket to fold or form a buckled region which can easily tear and develop a leak. This gets aggravated if there are any attempts to grip the PVC jacket to provide strain relief in the closure.
Another problem is that the tip cables are not always perfectly round in cross section because of the softness of the sheath. Also, the actual outside diameters of the cables vary somewhat depending on how the core conductors actually pack together during cable manufacture.
Thus, one object of this invention is to develop a sealing grommet for a splice case or the like, where the grommet can effect a gas tight seal about a soft-sheathed cable, and at the same time be sufficiently gentle on the cable so that the cable sheath will not tend to collapse or leak.
It is a second object that the sealing grommet permit easy cable insertion.
It is a third object that the sealing grommet be effective in gripping the cable for strain relief.
It is a fourth object that the sealing grommet allow for a range of dimensional variations in the cables sealed.
It is a fifth object that the sealing grommet be easy and inexpensive to manufacture.